The line closed in February 2010 between Gospel Oak and Stratford for the installation of a new signalling system and the extension of 30 platforms, ultimately to allow four-car trains to run on the London Overground network. The line reopened on 1 June 2010 with a reduced service and none on Sundays,[2] and with the upgrade work completed the full seven-day service resumed on 22 May 2011.

The North London Line between Richmond and North Woolwich derives from five connecting sections which were opened over 25 years from 1846:

The easternmost section opened as the Eastern Counties and Thames Junction Railway in 1846/7 between Stratford and North Woolwich. The later construction of the Royal Victoria Dock necessitated a swing-bridge on the original route south of Canning Town which was rerouted in 1850 via Custom House and the Connaught Tunnel. The original route was retained as the Silvertown Tramway, a local freight line connected at both ends to the new main line.

The main central section opened from 1850 to 1852 as the East & West India Docks & Birmingham Junction Railway (renamed the North London Railway (NLR) in 1853). This gave a link from the Euston main line near Primrose Hill to the docks at Poplar via Bow.

The line between Broad Street and both Kew Bridge and Richmond was electrified by the LNWR in 1916 on the 4th rail DC system.

In 1944 passenger service on the NLR Poplar branch ceased. Freight traffic continued on the branch until the docks on the Isle of Dogs until 1980. The trackbed of the southern part of the branch, from Poplar to Bow, was used for the Docklands Light Railway (DLR) branch to Stratford.

The service was listed for closure in the 1963 Beeching Report, with losses claimed as being £69,000 per year - £1.1 million in today's money. It was saved after a huge campaign. The line was Grant Aided under the 1986 Transport Act and was under threat when the Conservative Government of 1970-71 proposed to reduce Grant Aid funding. That threat - eventually removed - led to the founding of a new campaign group, the North London Line Committee which tried to work with British Rail management to promote the service.

In 1979 the North Woolwich to Stratford service was extended to Camden Road as the CrossTown LinkLine service using the same Cravens-built diesel multiple unit trains. There were no intermediate stations until in 1980 Hackney Wick, near the site of the former Victoria Park station, opened and Hackney Central re-opened, Homerton re-opened in 1985 (both those stations had closed in 1944). New platforms were built at West Ham for interchange with the adjacent Underground station.

In the 1980s Broad Street station closed and the Tottenham Hale–Stratford link and the station at Lea Bridge ceased to be used by regular passenger trains. The line between Dalston and North Woolwich was electrified on the third-rail system and Broad Street services were diverted to North Woolwich using former Southern Region 2-EPB types built in the 1950s. The two-car trains soon proved too small and were replaced by three-car Class 313 electric multiple units. The new service was branded by British Rail as the North London Link, and some signs using this name still exist.

In December 2006, as with the Poplar branch (see above), the line between Stratford and North Woolwich was permanently closed to make a way for a future DLR extension from Canning Town to Stratford International (opened February 2011). The section south of Canning Town was not used by the DLR as it is largely duplicated by the DLR King George V branch. Instead, the section will become part of Crossrail's branch to Abbey Wood (opening 2018). The section south of Stratford had always been the Cinderella end of the line, when there were operating problems it was common for trains to be turned short at Stratford.

Despite published performance figures[3] the North London Line used to be regarded by frequent travellers as offering a poor service,[4] with extremely congested trains and an unreliable service[5] with trains often cancelled shortly before they are due to arrive. A 2006 London Assembly report described the current service as "shabby, unreliable, unsafe and overcrowded", proposing the transfer of the service to Transport for London (TfL) as a solution to improve the quality of the service[6] due to upgrade plans[7] which coincided with the extension of the East London line. A report on the line can be found on the London Assembly website.[8]

In addition to the primary Broad Street - Richmond service there were services that linked Broad Street with Harrow & Wealdstone and Watford Junction on the West Coast Main Line. Most of these were routed via the line between South Hampstead and Camden Road, calling at Primrose Hill, although some travelled via Hampstead Heath and switched to the Watford line at Willesden Junction. By the time that Broad Street closed in 1986 the Watford services operated only in the rush hours, and they were diverted to Liverpool Street by way of a new link in Hackney, known as the Graham Road Curve. Trains were frequently cancelled as British Rail management needed to prioritise other services, and this led to falling patronage. British Rail applied to close the service in 1990 and the last trains ran two years later.

The AC electrification of the eastern part of the North London Line uses the previously unelectrified northern pair of tracks, which were also partially singled at the same time. Between Canonbury and Highbury & Islington there is a line which links to the East Coast Main Line at Finsbury Park. This used to carry passenger trains to / from various main line stations (such as Edgware, Alexandra Palace, High Barnet, Welwyn Garden City and others) over part of the North London Line to Broad Street Station; however, with the electrification of the Great Northern Electrics suburban lines in 1976, trains were diverted into Moorgate and London King's Cross stations, this link has only been used for freight trains. It too was singled concurrent with the AC electrification of the eastern part of the North London Line.

Most of the line runs in a curve across north London. Only Richmond and Kew Gardens stations at the western end are south of the River Thames. The river crossing is made by Kew Railway Bridge on tracks which are shared with the London Underground District line. The location of the eastern extremity has varied over the years. Between 1944 and 1986 it was at Broad Street station, then it switched to North Woolwich. It was cut back to Stratford. A tunnel, the Hampstead Heath tunnel, runs under Hampstead between Finchley Road & Frognal and Hampstead Heath. The line is double track throughout with a mix of triple and quadruple track between Camden Road and Dalston Kingsland. The former North Woolwich branch included a section of single track between Custom House and North Woolwich stations, and the Broad Street branch was at one time formed of quadruple track.

During the February–May 2010 blockade, the Caledonian Road & Barnsbury, Highbury & Islington and Canonbury stations were rebuilt to allow the extended East London line to serve Highbury & Islington on fully segregated tracks on the south side of the cutting. Under the reinstated four-track arrangement, the North London line moved to the north side of the cutting between Dalston Kingsland and Highbury and Islington, before switching to the inner pair of tracks towards the former Maiden Lane station, leaving the outer pair for freight use only.

Originally the line was electrified in 1920 using the 4th rail +420v/-210v system as used by London Underground. This was changed in the 1970s to +630v/0v, which requires no change to the trains, only to the substations. The trains (1957 stock at the time) were then simply modified for return current to be by the running rails, as on other third rail systems, and the 4th rail, and the centre shoes on the trains, was then removed. Four rail running was retained between Richmond and Gunnersbury for the Underground trains that share this section, with the centre rail bonded to the running rails. The line is now electrified using the third rail system from Richmond to Acton Central. Overhead lines are used from Acton Central to Stratford. The line to Broad Street used third rail supply and when the through service to North Woolwich started in 1985 trains used the third rail throughout. When the trains were replaced a few years later by dual-voltage Class 313 trains it became possible to use the overhead line equipment which had been added to parts of the line for the benefit of freight trains. There had been some unexpected difficulties with earth currents from the third rail system which this overcame. This use was steadily extended and trains had to make a number of changes between traction current supplies during their short journey. These were at Hackney Wick, Dalston Kingsland, Camden Road and Acton Central. With the final upgrade of the line between Camden Road and Stratford, the need to change traction current systems on this stretch was eliminated, and now the only changeover takes place at Acton Central.

The line crosses or comes into contact with a very large number of railway lines, especially lines radiating from central London. This does provide opportunities to move between separate sectors of suburban London without having to enter the central zone.

at North Woolwich passengers could cross the Thames via the Woolwich foot tunnel or the free ferry to Woolwich Arsenal station for connections on the North Kent line to Medway Towns, Gravesend, Dartford, Sidcup, Abbey Wood, Blackheath, Lewisham, Greenwich and Central London. This service is now provided by King George V DLR station which offers direct rail services to Woolwich Arsenal from North Woolwich, providing quick interchange to services on the North Kent line.

Between 1955 and 1985 the North London Line passenger services were operated by Class 501 units which were based at Croxley Green depot near Watford. These also operated an all stations service between Watford Junction and Euston. These were succeeded by Southern Regions Class 416 EMU for a short period with these units being allocated to Selhurst depot in south London.

These were succeeded by Class 313 EMUs which worked the route until 2010.

London Overground (the operator at this date) purchased Class 378 Capitalstar four-car dual-voltage electric trains, compatible with both 750 V DC third-rail and 25 kV AC overhead power sources,[9] and a number of these units are now operating in 5-car formations to address the additional demand on the route.

Trains run seven days a week day from approximately 06:00 (09:00 Sundays) until 23:30. During peak times, there are four trains per hour between Richmond and Stratford; and four trains per hour operate between Clapham Junction and Stratford on the West London Line service, making up a total of eight trains per hour between Willesden Junction and Stratford. During off-peak times, there are four trains per hour between Richmond and Stratford, and four trains per hour on the West London line between Clapham Junction and Willesden Junction, with two of these per hour continuing to Stratford, making up a total of six trains per hour between Willesden Junction and Stratford.

The introduction of the new four-car, air-conditioned trains, combined with improved signalling and passenger information has dramatically overhauled the service, making it a very effective alternative to travelling through central London for many orbital journeys.

Transport for London has extended platforms at some stations along the route to prepare the line for 5-car operation in 2015. This project aims to combat overcrowding on the line. The first 5-car trains started in Summer 2015 and now the project has been completed successfully.

Map of the London Overground network, with the NLL connected to other lines

The planned link between the proposed High Speed 2 line and the existing High Speed 1 line would have used the North London Line alignment around Camden Road station which may have reduced the existing or future capacity of the line. Due to its heavy investment in the line and the passenger growth on it, Transport for London is against the alignment's use as a link between the two High Speed lines.[16] This link has now been removed from the parliamentary bill.

The extension closed on 30 June 1986, but although the track was lifted the viaduct remained in place. In 2010, the route was re-opened as part of the extended East London Line, which, like the North London Line, is operated by London Overground.